1. Field of the Invention
The present invention relates to a dichroic mirror for separating or synthesizing light having a plurality of wavelengths which is comprised of a multilayered film, and optical apparatus and detecting method using the dichroic mirror.
2. Description of the Related Art
Conventionally, an apparatus shown in FIG. 1 is known as an optical apparatus for projecting or receiving light in the same direction after separating or synthesizing light having a plurality of wavelengths. This optical apparatus is arranged such that rays of light from two light sources 1a and 1b having different wavelengths (wavelengths .lambda.1 and .lambda.2) are converted into parallel rays of light by means of collimator lenses 2a and 2b, respectively, and the rays of light are reflected or transmitted by using a dichroic mirror 2, which is an optical element, owing to differences in the wavelength, thereby synthesizing the light. FIG. 2 shows a spectral characteristic of the dichroic mirror, and if .lambda.1 is set in a reflected-wavelength region (e.g., 700 nm) and .lambda.2 in a transmitted-wavelength region (e.g., 900 nm), the two light waves are synthesized by the dichroic mirror, making it possible to project the light with the two wavelengths (.lambda.1 and .lambda.2).
In addition, as an optical filter used in this type of optical apparatus, one having a multilayered structure, called the dichroic mirror, is conventionally known. As shown in FIG. 3B, the film of the conventional dichroic mirror is comprised of, for instance, a substrate (refractive index n=1.5)/0.40H, 0.96L, 0.94H, 0.85L, (1H, 1L)8, 1.01H, 1.05L, 0.45H/air (n=1). The dichroic mirror of this arrangement has the wavelength-transmittance characteristic such as the one shown in FIG. 3A. The aforementioned values represent optical film layer thicknesses (nd) in a case where 1/4 is set to be 1 when 1=750 nm, and "H" and "L" respectively represent a material layer having a high refractive index and a material layer having a low refractive index (e.g., the former being 2.26 and the latter 1.46). As examples of these material layers, it is possible to cite TiO.sub.2 and SiO.sub.2. In addition, (1H, 1L)8 represents a group of repeatedly laminated material layers, indicating that 1H and 1L are repeated 8 times.
With the above-described conventional optical apparatus, however, since a dichroic mirror 5 (FIG. 1) is disposed in the paths of parallel rays of light, there were problems including a decline in the economic efficiency due to the tendency toward a large-size apparatus and an increase in the number of component parts such as the collimator lenses.
In addition, as for the characteristic of the dichroic mirror serving as the optical filter, the steeper the rising characteristic shown in FIG. 3A, the light having the closer two wavelengths can be separated or synthesized. An example in which the light with two wavelengths is synthesized by the dichroic mirror is shown in FIG. 4. In this case, the characteristic required of the dichroic mirror is that, as shown in FIG. 5, the reflectance is high (the transmittance is low) at the wavelength .lambda.1, and the transmittance is high at the wavelength .lambda.2. In the dichroic mirror having such a characteristic, the power of the light with .lambda.1 and the power of the light with .lambda.2 can be synthesized efficiently into optical power after synthesis. Also, a case where the light is separated on the basis of the difference in wavelength can be considered in utterly the same way. Accordingly, the steeper the rising characteristic, the closer the reflecting region and the transmitting region in FIG. 5 are, and it is possible to separate or synthesize the light having two closer wavelengths.
As methods of obtaining a steep rising characteristic when the angle of incidence is small, it is possible to cite, among others, the following methods: (1) the difference between refractive indices of two adjacent material layers is made large (e.g., TiO.sub.2 (n=2.26) and SiO.sub.2 (n=1.46)), and (2) the number of repeated times for the group of repeated layers is increased. In addition, when the angle of incidence is large, in a case where the angle of incidence is 70.degree., as shown in FIG. 3A, the transmittance characteristic differs between P polarized light and S polarized light. Hence, in the case of light such as LED light, the light is randomly polarized light in which a P polarized component and an S polarized component are randomly mixed, and since the transmittance characteristic becomes the average of the P polarized light and the S polarized light, so that a ripple occurs in the vicinity of a 50% transmitting region. Since the characteristic of the randomly polarized light becomes an average value of the characteristic of the P polarized light and the characteristic of the S polarized light, a ripple occurs where the transmittance is in the vicinity of 50% (in terms of the wavelength, in the vicinity of 740 nm in FIG. 6). It should be noted that FIG. 6 shows the characteristic in the case where the angle of incidence of the dichroic mirror is 70.sup.20 in the conventional arrangement of the film. The greater the difference in the refractive index and the greater the frequency of repetition, the more noticeable the difference between the P polarized light and the S polarized light becomes. Since a long wavelength range is required for the transmittance to rise from a low wavelength region to a high wavelength region as a result of this ripple, it becomes difficult to synthesize or separate the light having two close wavelengths.
As measures for solving the above-described problem, the following measures, among others, are conceivable: (1) an optimum frequency is selected as the frequency of repetition, and (2) the two material layers are selected such that the difference in the refractive index becomes small. With respect to the measure (2), if the difference in the refractive index is made small, although the rising characteristic, to which the group of repeated layers is related, can be improved for the above-described reason, there occurs a drawback with respect to layers (referred to as adjustment layers) other than the group of repeated layers. Namely, although a wider wavelength region between "A" and "B" (i.e., a nontransmitting band), which is shown in FIG. 3, is desired (since the wavelength spectrum characteristic is wide to a certain extent in the case of a light source such as an LED), if the difference between the refractive indices of the two material layers in the adjustment layer is made small (e.g., in the case of TiO.sub.2 and Al.sub.2 O.sub.3 shown in FIG. 18 which will be referred to later), the nontransmitting band becomes narrow. Hence, it becomes impossible to efficiently separate or synthesize the light having two wavelengths.
Here, a description will be given of the nontransmitting band located between "A" and "B." The nontransmitting band is similar to the reflecting band shown in FIG. 5, and is related to the light reflected by the dichroic mirror. If consideration is given to a case in which the light with the wavelength .lambda.1 is reflected in FIGS. 4 and 5 referred to above, in the case of the conventional dichroic mirror (the angle of incidence: 45.degree.) shown in FIG. 7, a wavelength falling between B (approx. 610 nm) to A (approx. 750 nm) can be selected as .lambda.1. If the distance between B and A is short, the range from which the wavelength can be selected becomes the narrower. Meanwhile, if light such as that of the LED is used as a light source, the wavelength spectrum is wide to a certain extent; therefore, if the distance between B and A is short, not all the light emitted from the LED can be reflected, resulting in a power loss (not all the light other than the light having wavelengths falling between B and A is reflected, and that light is transmitted in accordance with its transmittance). Accordingly, unless the distance between B and A is wide to a certain extent, it is impossible to efficiently separate or synthesize light with two wavelengths.